<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
    <style>
      body,
      ul {
        margin: 0;
        padding: 0;
      }
      body {
        width: 1920px;
        height: 1080px;
        box-sizing: border-box;
        border: 3px solid red;
      }

      ul {
        width: 100%;
        height: 100%;
        list-style: none;

        display: flex;
        flex-direction: row;
        flex-wrap: wrap;
      }

      li {
        width: 33.333%;
        height: 50%;
        box-sizing: border-box;
        border: 2px solid green;
        font-size: 30px;
      }
    </style>
  </head>
  <body>
    <ul>
      <li>1</li>
      <li>2</li>
      <li>3</li>
      <li>4</li>
      <li>5</li>
      <li>6</li>
    </ul>
  </body>
</html>
